A. ACIS T_GAIN Epoch 55 (August-October 2013)

The periodic ACIS T_GAIN correction upgrade for August-October 2013 (Epoch 55) has
been released. The corrections are at nominal/expected levels of less
than 2% of the energy value. This upgrade as usual affects
observation data taken over the two most recent T_GAIN Epochs (55 and 54), and so
would update PHAs for OBS_IDs taken since 01 May 2013. Users with such datasets are
encouraged to revalidate their analysis results after reprocessing
their data with the upgraded CalDB files. For specific details of the
new calibrations in this release, see the technical details section below.

PIPELINES/TOOLS AFFECTED:

THREADS AFFECTED:

B. ACIS CONTAM Version N0008

Location:

$CALDB/data/chandra/acis/contam/

Filename:

acisD1999-08-13contamN0008.fits

The ACIS Optical Blocking Filter (OBF) contaminant calibration file requires an update due to recent
changes the depth and composition of the contaminant with time. New modeling results from cosmic sources
galaxy cluster Abell 1795 and blazar MKN421, in conjunction with fits of SNR EO1O2-72 calibration
observations have resulted in a new ACIS contamination model.

The rate of deposition has increased markedly since mid 2009; The cause of this is not certain.
However, the shape of deposition curve since 2009 has recently shown a more linear behaviour with time
than was previously included in the most recently released CONTAM file model (version N0007, May 2012,
CalDB 4.4.10.)

Regarding composition, it appears that the concentration of the several component elements in the
contamination layer are also changing, which requires time-varying adjustment of
the optical depths for carbon (C-K) and fluorine (F-K) with respect to oxygen (O-K),
and a modified spatial model for ACIS-S in
particular. A purely elemental model with the appropriate time- and spatially-dependent adjustments
has recently been derived, and is being released here.

Caveat to
users: Beware when using high ChipY locations on ACIS-S3 for imaging spectroscopy with
the CONTAM N0008 model! We are somewhat uncertain of the contaminant layer depth at high chipy
for ACIS-S, and fitting results may be compromised at this location. For technical details,
see Section II.B. below.

PIPELINES/TOOLS AFFECTED:

THREADS AFFECTED:

III. TECHNICAL DETAILS

A. ACIS T_GAIN Epoch 55 (August-October 2013)

The ACIS time-dependent gain corrections (T_GAIN) have recently been updated for current changes from the previous T_GAIN epoch, specifically Epoch 55, which was August-October 2013. With the addition of these new corrections, derived from ACIS External Cal Source (ECS) data taken during radiation zone passes, the CalDB files extending from May-August 2013 (i.e. Epoch 54) have been finalized, and new non-interpolating T_GAIN files are now implemented for Epoch 55.

The magnitudes (in eV) of the new gain corrections, versus photon energy, are given in Figs. 1-3 below. The corrections are of the usual order in magnitude, specifically less than 2.0% of the photon energy value. Fig. 1 below gives the corrections for the ACIS-I aimpoint on chip ACIS-3, for the CTI-corrected case, which is the only one applicable to FI chips. Figure 2 gives the corrections for the ACIS-S aimpoint on ACIS-7, for the case where the BI chips are CTI-corrected. Finally Fig. 3 gives the corrections for ACIS-7 for NON-CTI-corrected BI chips. These would be relevant to GRADED DATAMODE observations with ACIS-S, for example.

Long term trends for chips I3 and S3 are plotted below in Figure
4a and b, through Epoch 55, the latest release. The chip locations for these
plots are given in the respective labels above the plots. Note that
the locations are not the aimpoint positions, as with Figs. 1-3 above, but
are near the centers of each chip.

Figs 4a and 4b: Long term gain trends for specific
ECS line energies from Epoch 1 to Epoch 55 inclusive, and for the
positions on the respective chips given in the figure
labels. (Click on images to expand the plots.)

B. ACIS CONTAM Version N0008

The deposition rate and composition of the contaminating layer on the ACIS Optical Blocking
Filter (OBF) continues to change and require adjustments to the model used to estimate its absorption
effects. With this new N0008 release, for the first time there is a systematic, purely elemental model
(no edgeless "fluffium" component) for the ACIS contamination. This has been made possible by varying
not only the depth, but the chemical composition of the contaminant with time and location.

Figure 5 below gives the estimated optical depth (tau) of the contaminant versus mission date.
Note the change in the deposition rates throughout the mission, and in particular the abruptly upward,
almost linear, increase in the contamination rate beginning in mid 2009. Fig. 6 gives the change in
tau between the center and the edge of the detector, which shows that the increasing spatial
variation of the contaminant with time.

Fig. 5: The optical depth "tau" of the contaminating layer on the ACIS OBF as modeled
versus year of the mission time line. Note that the trend since y2009 has been essentially
a linear increase with time.

Fig. 6: This figure shows that difference in the optical
depth at 700 eV between the edge and center of the detector.

Fig. 7 below shows the accumulation rates for carbon and oxygen. In particular for the
period 2003-2005, the composition of the contaminant appears to have been changing with time.

Fig. 7: The Carbon and Oxygen deposition rates versus year, showing the
variation in the composition of the contaminating layer.

While the most recent (2013) data taken to monitor the contaminating layer indicate the
layer depth is almost linearly increasing with time since 2009, the previous model (N0007)
anticipates a leveling-off of the contamination layer going forward. Hence a new model is
needed properly to account for the current trend. Upgrading the model for the full mission
means that the new CONTAM model is not identically backward compatible with the previous
version (N0007, released in CalDB 4.4.10, May 2012). However, the new model fits the tau
for all periods illustrated in Figs 5 and 6 above, and hence does not significantly affect
midmission results.

This is the procedure that has been employed in developing the new contamination model:

Develop a preliminary contamination model to fit Abell 1795 data at the centers of both ACIS-I
and ACIS-S arrays as a function of time. Add a spatial component appropriate for each array.

Using the preliminary model, fit LETG/ACIS-S spectra of blazars to constrain the optical
depths at the C-K, O-K, and F-K edges. The gratings data is primarily restricted to two
rows (167 and 32) on ACIS-S. (Fig. 8 below.)

Fit the raster scans of Abell 1795 observations using a purely elemental model, adopting edge
constraints measured from the gratings data, and determine the normalization of the contamination
model as a function of position and time.

Make final adjustments in tau(C-K) and tau(F-K) as a function of TIME, using LETG/ACIS-S data.
No adjustment is needed at the O-K edge. (Result: The new CalDB CONTAM file N0008)

Verify the model by fitting E0102-72 observations at various locations on ACIS-I3 and
ACIS-S3, using the Standard IACHEC E0102 Model. (See Fig 9a-f below.)

Hence, ACIS CONTAM version N0008 accomplishes two goals, (1) implementation of a more realistic model
(without the edgeless "fluffium" component used in previous models) and (2) more accurate representation
and prediction of current and future loss of effective ACIS QE due to the OBF contamination layer.

Fitting spectra from E0102-72 taken at various times during the mission (on ACIS-I
and ACIS-S) indicates that the CONTAM N0008 model is basically successful (Fig. 9a-f below.)
However, we note that there is one particular position on ACIS-S3 where the E0102-72 certain line
normalizations are skewed with respect to the general results, specifically in the high ChipY
location of Node 2. (See the green square symbol in each of Fig. 9b, c, and d.)

Caveat to
users: Beware when using high ChipY locations on ACIS-S3 for imaging spectroscopy with
the CONTAM N0008 model! We are somewhat uncertain of the contaminant layer depth at high chipy
for ACIS-S, and fitting results may be compromised at this location.

The overall affect of the increasing contamination layer is best illustrated using spectra from
Abell 1795, a stable source observed by the ACIS Calibration team annually since the start of the
mission. See Fig. 10, for an illustration of the loss of effective ACIS-S effective area with time.
These data have been used for the first step in modeling the contaminant layer with time.

Fig. 10: This figure shows the result of a simultaneous
fit to spectra extracted from nine observations of A1759 in 2000, 2002, 2004,
2005, 2009, 2010, 2011,2012 and 2013.

The comparative ACIS-I effective areas (EAs) for January 01 of 2000, 2004, 2009, and projected for 2015,
are given in Fig. 11 below. The solid curves are those calculated using CONTAM N0007; the dashed curves
of the corresponding color are for the same dates, but using the new CONTAM N0008 file. The early-
and mid-mission EAs are not much affected by the new model, except for very near the C-K, O-K, and F-K
absorption edges, as would be expected from the new elemental basis of the model. (Changes near these
edges do not affect users' fitting results significantly for mid-mission observations.)

Fig. 11: The ACIS-I aimpoint effective areas versus Energy for January 01 of 2000 (green),
2004 (blue), 2009 (cyan), and projected for 2015 (red). The solid curves give the calculation
using CONTAM N0007; the dashed curves give the same, but using CONTAM N0008.

The comparative ACIS-S effective areas (EAs) for January 01 of 2000, 2004, 2009, and projected for 2015,
are given in Fig. 12 below. The solid curves are those calculated using CONTAM N0007; the dashed curves
of the corresponding color are for the same dates, but using the new CONTAM N0008 file. The early-
and mid-mission EAs are not much affected by the new model, except for very near the C-K, O-K, and F-K
absorption edges, as would be expected from the new elemental basis of the model. (Changes near these
edges do not affect users' fitting results significantly for mid-mission observations.)

Fig. 12: The ACIS-S aimpoint effective areas versus Energy for January 01 of 2000 (green),
2004 (blue), 2009 (cyan), and projected for 2015 (red). The solid curves give the calculation
using CONTAM N0007; the dashed curves give the same, but using CONTAM N0008.

However, the more rapidly increasing contamination layer does indicate that ACIS-S is not optimal
for very soft sources in the future, particularly at and below the Carbon edge (283 eV). It is
suggested that LETG/HRC-S is a better configuration in this energy range going forward, with larger EAs
and better calibrations as well.

C. HRC-I GMAP & PIBGSPEC September 2013

A new HRC-I GMAP has been generated for the (UTC) period
2013-09-16T12:00 to the present, and extending into 2014. The
corrections were generated using G21.5-0.9 and HZ 43 calibration
observations taken regularly to monitor HRC-I, combined with a
sequence of ArLac observations at 21 specific locations on HRC-I, taken
annually.

The HRC-I gain continues to decline slowly (typically ≤5%
per year). Annual updates to the gain maps are found to be adequate
to maintain 2% consistency in the PI. Images of the new Sept 2013
GMAP, and a ratio of the new map to the previous (2012) one already
in CalDB are shown in Fig. 13A & B below. In Fig. 13B, the sense of the
ratio is that the 2013 map includes equal to or larger corrections
(equal to or greater than unity) compared with last year's gmap.

A corresponding upgrade to the PIBGSPEC library fo 2013 has also
been derived, effective for the same time-period as the above HRC-I
GMAP. The background PI spectra are generated from the same HRC-I
calibration observations of AR Lac used to derive the new GMAP.
These 21 observations are reprocessed with the new GMAP, generating
new PI scaled to SUMAMPS values.

For each year, the HRC-I Cal team extracts events in a 800x800
pixel box at the 17 observation locations that are within 10 arcmin
of the aim point, excluding the location of the source. They sum the
PI spectra from each location, weighting by exposure time, to create
the final PI spectrum for the year. Finally, the PI spectra are
Loess-smoothed with a third-degree polynomial fit over ranges 10
channels wide.

Matching CIAO and CALDB Versions

News

CalDB 4.6.7
an upgrade to CalDB 4.6.5 and later versions, has been released. Notable additions include
the ACIS T_GAIN Epoch 60 quarterly upgrades (Nov-Jan 2015), and a correction to the HRC index
file. See the CalDB 4.6.7
Release Notes for details.

16 Dec 2014

CalDB 4.6.5,
an upgrade to CalDB version 4.5.9 and later versions, has been released. Notable additions
included are ACIS T_GAIN Epoch 59 updates (Aug-Oct 2014), HRC-I GMAP and PIBGSPEC updates for 2014,
and HRC-S QE version N0013 and QEU (version N0007) modifications. See the
CalDB 4.6.5 Release
notes for details.

CalDB 4.6.3,
an upgrade to CalDB version 4.5.9 and later versions, has been released. Notable additions
included are ACIS T_GAIN Epoch 58 updates (May-July 2014), HRC-S QEU files for 2014-2020,
mitigation of the HRC-S QEU Caveats.
CalDB 4.6.3 Release
notes for details.

25 Jul 2014

HRC-S QEU File Search Error Caveat
added. This applies to all HRC-S observations taken in 1999 and in 2014. A workaround is
prescribed on this page. The fix will be released in mid August 2014. NOTE: These have been
mitigated with the CalDB 4.6.3 release on 29 Aug 2014!

09 Jul 2014

CalDB 4.6.2, an upgrade to CalDB version 4.5.9
and later versions, has been released. It contains (a) an ACIS quarterly T_GAIN
upgrade (Epoch 57, Feb-Apr 2014), (b) upgrades to PCAD CALALIGN files for 2003, 2006, 2012,
and 2013, and finally (c) an upgrade to the ACIS CONTAM file to improve on-axis calibration
performance from April 2013 onward. See the
CalDB 4.6.2 Release
notes for details.

09 Apr 2014

CalDB 4.6.1.1, a patch upgrade to CalDB 4.6.1
which corrects the HRC branch index listings of the latest two HRC-I background event lists
for 2012 and 2013, so that they are selected appropriately for OBS_IDs taken after
(UTC) 2012-01-01T00:00:00. See the
CalDB 4.6.1.1 Release
notes for details.

26 Mar 2014

CalDB 4.6.1, an upgrade to CalDB version 4.5.9
and later versions, has been released. It contains (a) an ACIS quarterly T_GAIN
upgrade (Epoch 56, Nov-Jan 2014), as well as (b) a new HRC-I BKGRND event list library
which adds new files for the four most recent calendar years (2010-2013). See the
CalDB 4.6.1 Release
notes for details.